Now there might be an explanation. A team of Israeli researchers at the Weizmann Institute report that men's sexual arousal is reduced after smelling women's tears — even when the crying woman isn't present. This suggests that tears send out a chemical signal which encode a subtle, or not so subtle, turn-off message to men.
Emotional tears, a uniquely human phenomenon, have long perplexed scientists, who have speculated about their evolutionary purpose. Noam Sobel, a neurobiologist who runs the "Olfaction Research Group" at Weizmann and is lead author of the paper published today in Science Express said in a statement (I tried unsuccessfully to reach him, so this is from the press release): "This study raises many interesting questions. What is the chemical involved? Do different kinds of emotional situations send different tear-encoded signals? Are womens tears different from, say, men's tears? Childrens tears? This study reinforces the idea that human chemical signals even ones were not conscious of affect the behavior of others."
The researchers say that a number of previous studies, for instance, found substances in human sweat can carry a range of emotional and other signals to those who smell them. So, they wondered, what types of signals tears of sadness might carry.
The researchers report:
Male volunteers sniffed either tears or a control saline solution, and then had these applied under their nostrils on a pad while they made various judgments regarding images of women's faces on a computer screen. The next day, the test was repeated — the men who were previously exposed to tears getting saline and vice versa. The tests were double blinded, meaning neither the men nor the researchers performing the trials knew what was on the pads. The researchers found that sniffing tears did not influence the men's estimates of sadness or empathy expressed in the faces. To their surprise, however, sniffing tears negatively affected the sex appeal attributed to the faces.
To further explore the finding, male volunteers watched emotional movies after similarly sniffing tears or saline. Throughout the movies, participants were asked to provide self-ratings of mood as they were being monitored for such physiological measures of arousal as skin temperature, heart rate, etc. Self-ratings showed that the subjects emotional responses to sad movies were no more negative when exposed to womens tears, and the men smelling tears showed no more empathy. They did, however, rate their sexual arousal a bit lower. The physiological measures, however, told a clearer story. These revealed a pronounced tear-induced drop in physiological measures of arousal, including a significant dip in testosterone a hormone related to sexual arousal.
Finally, Sobel and his team repeated the previous experiment within an fMRI machine that allowed them to measure brain activity. The scans revealed a significant reduction in activity levels in brain areas associated with sexual arousal after the subjects had sniffed tears.
Here's The New York Times take on the research.
This program aired on January 6, 2011. The audio for this program is not available.